Skyhooks and Space Elevators

A skyhook is a proposed space transportation concept that will help make spaceflight affordable to everyone.  When used as part of a combination launch system it will make the building of a spacefaring civilization possible on a commercial basis.  There are two kinds of skyhooks, a rotating skyhook, and a non-rotating skyhook.

A non-rotating skyhook is a much shorter version of the Earth surface to geostationary orbit Space Elevator that does not reach down to the surface of the Earth.  It is much lighter in mass, can be affordably built with existing materials and technology, and in its mature form, is cost competitive with what is thought to be realistically achievable using a Space Elevator, assuming materials strong enough to build a Space Elevator ever become available.  It works by starting from a relatively low altitude orbit and hanging a cable down to just above the Earth’s atmosphere.  Since the lower end of the cable is moving at less than orbital velocity for its altitude, a launch vehicle flying to the bottom of the non-rotating skyhook can carry a larger payload than it could otherwise carry to orbit.  When the non-rotating skyhook is long enough, Single Stage To Skyhook flight with a reusable launch vehicle becomes possible at a price that is affordable to just about anyone.

Another way to understand the non-rotating skyhook is to think of it as a momentum exchange device that consists of a space station in a higher altitude, higher energy, elliptical orbit, with a cable that hangs down to just above the atmosphere.  When a suborbital spacecraft coming up from the Earth docks at the lower end of the cable, it pulls the space station down into a slightly lower more circular orbit.  In effect, the space station gives up some of its energy to the arriving spacecraft so that the arriving suborbital spacecraft can stay in orbit instead of falling back to Earth.  When the spacecraft lets go of the lower end of the cable to return to Earth, it gives that energy back which allows the space station to return to a higher altitude, higher energy, more elliptical orbit.  The end result is that the energy that is exchanged between the non-rotating skyhook and the arriving spacecraft and then returned to the skyhook when the spacecraft departs, gets used over and over again every time a spacecraft makes a trip to the skyhook.  This exchange and reuse of energy reduces the amount of propellant the launch vehicle needs to carry which allows it to carry more payload.  Less propellant also makes for a smaller, lighter, and more affordable launch vehicle.  More payload means that the cost of the launch can be spread out over a larger amount of cargo.  Both of these changes reduce the cost per pound of getting to orbit.  When the skyhook cable is long enough, airliner like operations to space become possible at airliner like prices.

History

The idea for a non-rotating skyhook evolved from the idea of an orbital tower which was first proposed by Konstantin Tsiolkovsky back in 1895.  The orbital tower consists of a really tall tower that goes from the surface of the Earth all the way to geostationary orbit.  Its purpose was to provide an economical way of getting to orbit so that the human race could start building a spacefaring civilization.  The reason for wanting to build a spacefaring civilization was to avoid the projected collapse of our civilization at some time in the near future due to overpopulation.  A collapse that is considered by many to be inevitable if we remain a single planet species.  So why not use rockets?  Konstantin Tsiolkovsky knew about rockets.  After all, he is the person who first worked out the mathematics for using rockets to travel through space to other planets.  As a result of that work, he knew just how uneconomical chemical powered rockets are, which is why he wanted to find a better way of getting into orbit.   He got the idea for the orbital tower as a result of a trip to Paris, France where he saw the Eiffel Tower.  While he knew that such an orbital tower could not be built, he felt certain that the existence of a theoretical solution to the rocket problem would eventually lead to a real world solution that could be built.  He was right.

The idea of the orbital tower led to the creation of the space elevator concept, another idea that cannot be built.  That led to the idea of a rotating skyhook, a type of rotating space elevator that rotates in the plane of its orbit like a two spoke wheel rolling across the top of the atmosphere as it orbits the planet.  While this idea can be built with existing materials, it also has three very significant operational problems that have yet to be solved.

The first of these is the very short amount of time that is available for an arriving spacecraft to hook up with the end of the cable.  A rendezvous window that is literally only three to five seconds long.  This is what engineers and scientists like to call a “non-trivial problem.”

The second problem is maintaining the synchronization between the rotation rate of the skyhook with its orbital period.  Since the rotating skyhook is in an elliptical orbit, the rotation rate of the cable needs to be in sync with the amount of time it takes to orbit the Earth so that the lower end of the cable will be at the bottom of its swing when the rotating skyhook is at the low point of its elliptical orbit.  When a spacecraft docks with the lower end of the rotating skyhook at the low point of its orbit, it pulls the skyhook down into a lower orbit with a shorter orbital period.  Since the rotation rate of the cable does not change when the rendezvous occurs, the rotation rate of the rotating skyhook is now out of sync with the new orbit.  The rotating cable will need to be brought back into sync with the orbit before another spacecraft can use the system.  This is another non-trivial problem.

The third problem with the rotating skyhook has to do with how the release orbit of the spacecraft occurs one-half a rotation after a spacecraft docks with the cable at the bottom of its swing.  This linkage of the release orbit to the time of arrival causes a problem in that only a very small percentage of the release orbits will be pointed in the right direction for a spacecraft that is going to the Moon and beyond.  The only solution to this is to limit the departure speed of the spacecraft to a speed that will take it to a higher altitude orbit where the spacecraft will use its onboard propellant to circularize its orbit and wait until it is in the correct position to boost for its final destination.  This noticeably limits the usefulness and cost advantage of the rotating skyhook for manned spaceflights to the Moon and beyond.

It was the search for a workable solution to all these problems that led to the creation of the non-rotating skyhook.  A skyhook that can be affordably built and operated with existing materials and technology and that doesn’t have the problems of the rotating skyhook.

For more detailed information about what a non-rotating skyhook is and how it works, go here.

A 200-kilometer long basic Non-rotating Skyhook configured to receive a suborbital spacecraft coming up from the Earth.

 

Index of Articles

  1. Opening the High Frontier
  2. Skyhook, a Journey to Orbit and Beyond
  3. In the Beginning . . .
  4. Why do Rockets Cost so Much?
  5. Combination Launch Systems
  6. It’s All About Speed!
  7. Visions of the Future
  8. The Call of an Unlimited Future
  9. Combination Launch Systems, part 2
  10. Outward Bound: Beyond Low Earth Orbit
  11. and someday . . . Starships!
  12. Mars: how to get there
  13. Outpost Space Stations
  14. Dreams of Space
  15. The Moon or Mars?
  16. Skyhooks and Space Elevators
  17. Stratolaunch and the X-15
  18. Starship Congress
  19. Making Spaceflight Affordable
  20. How a Combination Launch System Works

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1 thought on “Skyhooks and Space Elevators”

  1. I like that you mention such elevators can use ion propulsion. Momentum can be built up gradually. And a skyhook’s catch or release is a sudden event — it is even more impulsive than a chemical. It’s a way to enjoy ion ISP and a good Oberth benefit.

    Besides great ISP it seems to me sky hooks can also save reaction mass by momentum exchange. If it catches payloads from higher orbits as well as lower orbits, momentum boosting maneuvers can be balanced with momentum sapping maneuvers and the sky hook’s orbit can be maintained with less reaction mass. I am thinking water rich asteroids in a loose lunar orbit could send propellent deeper into earth’s gravity well and thus provide up momentum. Propellent from the moon’s cold traps might be another source of up momentum for sky hooks in earth orbits.

    What do you think of non rotating sky hooks anchored from Deimos and Phobos? The moons are solid momentum banks that would enable virtually unlimited catches and throws without damaging the sky hook’s orbit. Between the tethers of the two moons is a Zero Relative Velocity Transfer Orbit (ZRVTO) so the moons could exchange payloads while using very little reaction mass.

    A Mars ascent/descent vehicle dropped from a Phobos tether would have less EDL issues than a craft incoming from a Earth to Mars Hohmann. Especially if Phobos could be used a propellent source.

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